Simplify the expression. $(-3r+7)(3r+1)$
Solution: First distribute the ${-3r+7}$ onto the ${3r}$ and ${1}$ $ = {3r}({-3r+7}) + {1}({-3r+7})$ Then distribute the ${3r}.$ $ = ({3r} \times {-3r}) + ({3r} \times {7}) + {1}({-3r+7})$ $ = -9r^{2} + 21r + {1}({-3r+7})$ Then distribute the ${1}$ $ = -9r^{2} + 21r + ({1} \times {-3r}) + ({1} \times {7})$ $ = -9r^{2} + 21r - 3r + 7$ Finally, combine the $x$ terms. $ = -9r^{2} + 18r + 7$